Elevated-temperature fatigue behavior and microstructure based cumulative damage evaluation of additive manufacturing superalloy under variable amplitude loading

Fatigue properties under service conditions are a critical barrier to the reliable application of additive manufacturing (AM) metals. Yet, the associated damage mechanisms and life evaluation approaches, particularly at long term, elevated temperature and variable amplitude (VA) loading, are almost unclear. To address these, high and very-high cycle fatigue VA tests and meso-microscale analyses were performed to investigate damage mechanism of a laser powder bed fused superalloy with heat treatment at service temperature of 650 °C, and a microstructure based cumulative damage evaluation approach was proposed. Results show that interior failures characterized by defect-assisted faceted cracking are predominant. VA loading tends to sequentially activate multiple defects, resulting in competitive multi-site crack nucleation. Increased stress levels accelerate crack growth, leading to the formation of localized rough growth areas and crack deflection. Both primary and secondary cracks grow transgranularly, with crack paths showing negligible dependence on grain orientation. The interior crack nucleation and growth mechanisms under VA loading are elucidated. A cumulative damage evaluation model incorporating the remaining life factor, correlation function transformation, and a reconstructed stress-life relationship was developed, with the prediction results being in close accord with the experimental data under VA loading. These findings provide new insights into the interior crack nucleation and growth mechanisms in AM superalloys and offer a predictive framework for fatigue life estimation under realistic service conditions.